Proton magnetometer
（重定向自Proton precession magnetometer）
![Proton magnetometer from 1967.](/Images/godic/202502/04/1967_Proton_magnetometer0425.jpg")
The proton magnetometer, also known as the proton precession magnetometer (PPM), uses the principle of Earth's field nuclear magnetic resonance (EFNMR) to measure very small variations in the Earth's magnetic field, allowing ferrous objects on land and at sea to be detected.